1. Field of the Invention
The present invention relates to gas turbine engine combustors which have a cyclonic pro-combustion mixing chamber (prechamber) and more particularly to such a cyclonic prechamber having a centerbody optimally designed for preventing backflash and autoignition.
2. Description of Related Art
Gas turbine engine combustors such as those used on aircraft and for ground based industrial and marine applications and which are liquid fueled are prone to undesirable high formation of nitrogen oxides (NO.sub.x) particularly from the standpoint of increasingly rigid government standards. As more advanced aircraft are built with evermore stringent requirements, low NO.sub.x combustors become even more important. Combustors have been designed that prevaporize the liquid fuel, i.e. convert to vapor or gaseous form prior to combustion, and the heat for fuel vaporization is provided by hot air supplied to the combustor. However, in many of the proposed combustors the liquid fuel residence time required for a minimally acceptable extent of prevaporiztion has required an inordinately large prechamber, which can result in autoignition therein of the fuel/air mixture during operation.
One solution to this problem is disclosed in U.S. Pat. No. 4,246,757 which issued Jan. 27, 1981, entitled "Combustor Including A Cyclone Prechamber And Combustion Process For Gas Turbines Fired With Liquid Fuel", by the present inventor and assigned to the present assignee. This patent discloses a combustion apparatus and process which provides a good balance between sufficiently long residence time of liquid fuel in a cyclonic prechamber or prevaporization zone for effective fuel vaporization therein and sufficiently short residence time of the ignitable mixture of the resulting fuel vapor with hot air as to substantially avoid autoignition in a cyclonic pre-combustion mixing chamber, which is conventionally referred to as a prechamber, and minimize flashback from a downstream combustion chamber over a variety of operating conditions.
Another U.S. Pat. No. 3,808,803, which issued May 7, 1974, and is entitled "ANTICARBON DEVICE FOR THE SCROLL FUEL CARBURETOR" by Enrico Salvi discloses a scroll-type carburetor having a frustro-conical plug, for preventing flashback, mounted on the upstream wall of a spin chamber at a location along the axis of the core outlet and the primary vortex. The spin chamber is scrolled, thereby providing a spiral passage ending in an annular array of swirl vanes located radially inward of the outer annular wall of the spin chamber. The spin chamber however does not have the capability of retaining the liquid fuel until it fully vaporizes.
U.S. Pat. No. 5,165,241, which issued Nov. 24, 1992, and is entitled "Air Fuel Mixer For Gas Turbine Combustor" discloses an air fuel mixer for a gas turbine combustor having a mixing duct, a set of inner and outer counter-rotating swirlers at the upstream end of the mixing duct, and a fuel nozzle located axially along and forming a centerbody of the mixing duct. High pressure air from a compressor is injected into the mixing duct through the swirlers to form an intense shear region and fuel is injected into the mixing duct from the fuel nozzle so that the high pressure air and the fuel is uniformly mixed when the fuel/air mixture is exhausted out the downstream end of the mixing duct into the combustor, ignited, and burned so as to produce minimal formation of pollutants. This mixer is designed to avoid flashback, whereby the flame of the combustor is drawn back into the mixing section, and to avoid auto-ignition of the fuel/air mixture which can occur within the premixing duct if the velocity of the air flow is not fast enough, i.e., where there is a local region of high residence time. One problem with the device is its length which is not as short as is desired particularly for use in aircraft gas turbine engines and more particularly for use in high speed supersonic aircraft gas turbine engines. It is also very desirable to have a mixing of air and vaporized liquid fuel prior to ignition and the apparatus of U.S. Pat. No. 5,165,241 does not appear to accomplish this.
The prechamber assembly of the present invention provides a long residence time for liquid fuel to be vaporized and a short residence time for the resulting mixture of fuel vapor and air which in turn results in minimal formation of nitrogen oxides (NO.sub.x). The present invention is further designed to prevent flashback by preventing the recirculating flows from reaching into the prechamber assembly. These are problems that are particularly acute in prechamber assemblies with strong cyclonic action because of the low pressures created by the strong cyclonic action.